Sulphonic fluorinated ionomers

ABSTRACT

Sulphonic fluorinated ionomers crosslinked by radical crosslinking of: A) crosslinkable sulphonic fluorinated ionomers, having equivalent weight 380-1300 g/eq and comprising: from 48% to 85% by moles of monomeric units deriving from tetrafluoroethylene (TFE); 
         from 15% to 47% by moles of fluorinated monomeric units containing sulphonyl groups —SO 2 F; from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula:  
                 
 
wherein: m=2-10, preferably 4-8; R 1 , R 2 , R 5 , R 6 , equal to or different from each other, are H or C 1 -C 5  alkyl groups; 
B) a fluorinated compound as crosslinking radical initiator; C) a fluorinated bis-olefin of structure (I) as crosslinking agent; the radical crosslinking being carried out at a temperature in the range 250° C.-310° C., preferaably 260° C.-300° C.

The present invention relates to sulphonic fluorinated ionomerscrosslinked by radical route suitable for the preparation of membranesfor electrochemical applications, in particular for fuel cells, and asionic exchange resins acting as catalysts.

Specifically, the invention relates to sulphonic fluorinated ionomerscrosslinked by radical route characterized by a high hydration degree,both at room temperature and at high temperature (up to 180° C.),without substantially compromising the physical integrity of theobtained membranes, wherefore they are usable also at high temperaturesin the range of 120° C.-180° C.

It is known in the prior art the use of the class of polymers called bythe term “ionomers” in electrochemical applications, such for example infuel cells, chlorosoda cells, lithium batteries and in reactors in whichthe ionomer acts as a solid catalyst. These applications implies thecontact of the ionomer with a liquid, in particular water, havingaffinity with the ionic functional groups of the ionomer itself.

Generally, the larger the amount of ionic groups present in the chain,the better the efficiency of the ionomer application, both in terms ofcapability of ionic exchange in electrochemical applications, and interms of catalyst activity in catalysis applications. From this point ofview, an important parameter is the equivalent weight of this ionomer.The lower the equivalent weight, the higher the percentage of sulphonicgroups present in the chain. Therefore, ionomers having a low equivalentweight are desirable since they give a higher application efficiency.

In electrochemical applications, for example in fuel cells, there is adirect correlation between the polymer conductivity and the waterretention from the ionomer itself. The ionic conductivity of thepolymer, besides being increased by the greater presence of ionic groupsin the polymer, results increased, within an upper limit, also by thelarger amount of water that the polymer is capable to retain (swellingdegree). However, the excessive affinity of the ionomer with water hasas a consequence the drawback of an excessive polymer swelling, whichassumes a gelatinous state consequently losing its physical integrity.The ionomer becomes therefore completely unusable in all theapplications wherein it is required under a solid form.

Also in the applications wherein the ionomer is mixed with or depositedon a support material, suitable to guarantee the shape and the physicalintegrity of the final membrane, the ionomer must however show aphysical consistency sufficient to prevent the release thereof from thesupport and it must be insoluble in the liquid medium with which itcomes into contact during the use.

Besides, the ionomeric membrane must be activated before the use,wherefore the chemical transformation of the precursor groups —SO₂F intothe corresponding sulphonic groups —SO₃H is necessary. The membraneactivation is carried out first by contacting it with an alkalineaqueous solution and then with an acid solution. During thistransformation phase, if the ionomer has a high swelling degree, it canpartially or completely dissolve in the reaction medium. At this point,it is impossible to recover the ionomer and separate it from the otherproducts of the transformation reaction.

In the prior art, to obtain a limited ionomer hydration and sufficientphysical integrity, polymers having a high equivalent weight, of theorder of 1,000-1,200, are used i.e. having a low concentration ofsulphonic groups, which represent the hydrophilic part of the polymer.Therefore, ionomers having a high equivalent weight absorb a limitedamount of water, which guarantees the polymer insolubility. On the otherhand, having few ionic groups, they have the drawback to give membraneshaving a lower ionic conductivity during the application. An example ofsaid ionomers is represented by the commercial product NAFION®, used infuel cells and having an equivalent weight of the order of 1,000-1,100.The membranes obtained from said ionomers have good mechanicalproperties. However, if these membranes are used at temperatures higherthan 100° C., the interstitial water, which is the carrier of theprotons H⁺ in fuel cells, tends to reduce itself, wherefore the membranetends to dehydrate and the membrane conductivity is drastically reduced.As a consequence, the membranes obtained by NAFION® are not efficientlyusable at temperatures higher than 100° C.

U.S. Pat. No. 4,940,525 describes sulphonic ionomers having a lowequivalent weight, lower than 725, used to obtain unsupported thickmembranes for fuel cells, only if the hydration product of the polymeris low, lower than 22,000. So low hydration values are indeed necessaryfor maintaining the physical integrity of the ionomer having equivalentweights lower than 725, provided that the equivalent weight is not lowerthan 500 (col. 6, 8-16). Therefore, according to the description of thispatent, it is impossible to obtain sulphonic ionomers of equivalentweight lower than 500 having the property of the insolubility in water.Besides, no mention is made to the behavior of the membranes at hightemperatures, of the order of 120° C.-160° C.

The need was felt to have available sulphonic fluorinated ionomers suchthat the obtained membranes have a high hydration percentage, both atroom temperture and at high temperature (up to about 180° C.) withoutsubstantially compromising the membrane physical integrity, whereforethe membranes are usable also at high temperatures, of the order of 120°C.-180° C., in electrochemical applications. Said membranes can be usedalso as ionic exchange resins.

An object of the present invention are therefore crosslinked sulphonicfluorinated ionomers obtainable by radical crosslinking of:

-   -   A) crosslinkable sulphonic fluorinated ionomers, having        equivalent weight 380-1300 g/eq, preferably 380-800 g/eq, and        comprising:        -   from 48% to 85% by moles of monomeric units deriving from            tetrafluoroethylene (TFE);        -   from 15% to 47W by moles of fluorinated monomeric units            containing sulphonyl groups —SO₂F;        -   from 0.01% to 5% by moles of monomeric units deriving from a            bis-olefin of formula:        -   wherein: m=2-10, preferably 4-8;        -   R₁, R₂, R₅, R₆, equal to or different from each other, are H            or C₁-C₅ alkyl groups;    -   B) a fluorinated compound as crosslinking radical initiator;    -   C) a fluorinated bis-olefin of the above structure (I) as        crosslinking agent;    -   the radical crosslinking being carried out at a temperature in        the range 250° C.-310° C., preferably 260° C.-300° C.

Among the fluorinated monomers containing sulphonyl groups —SO₂F we canmention:

-   -   F₂C═CF—O—CF₂—CF₂—SO₂F;    -   F₂C═CF—O—[CF₂—CXF—O]_(n′)—CF₂—CF₂—SO₂F        -   wherein X═Cl,F or CF₃; n′=1-10;    -   F₂C═CF—O—CF₂—CF₂—CF₂—SO₂F (vinylsulphonylfluoride);    -   F₂C═CF—Ar—SO₂F wherein Ar is an aryl ring.

Preferably the crosslinkable fluorinated sulphonic ionomers A) comprise:

-   -   from 54% to 71% by moles of monomeric units deriving from TFE;    -   from 45% to 28% by moles of monomeric units deriving from        vinylsulphonylfluoride F₂C═CF—O—CF₂—CF₂—SO₂F;    -   an amount higher than 0.4% by moles up to 3% by moles, more        preferably from 1% to 2.5% by moles of monomeric units deriving        from the bis-olefin of formula (I).

As regards the radical initiators B) used in the radical crosslinking ofthe present invention, they are selected from:

-   -   (d) branched perfluoroalkanes of formula:        C_(a)F_(2a+2) wherein a=5-15, preferably 7-11;    -   (e) halogenated compounds of formula:        ClO₂S(CF₂)_(n)SO₂Cl wherein n=4-10;    -   (f) peroxidic perfluoropolyether compounds having oxidizing        power in the range 0.8-6, preferably 1-3.5, of structure        T-O—(R_(f))—(O)_(c)-T′        -   wherein:            T,T′=—CF₃, —CF₂CF₃, —CF₂CF₂CF₃        -   c is an integer such as to give the above oxidizing power;        -   R_(f) perfluoropolyether chain having a number average            molecular weight in the range 1,000 and 30,000, preferably            4,000-20,000, comprising one or more of the following units:            —(CF₂O)—, —(CF₂CF₂O)—, —(CF₂CF₂CF₂O)—, —(C₃F₆O)—

Preferably the perfluoropolyether chain R_(f) is selected from thefollowing structures:—(CF₂O)_(a′)(CF₂CF₂O)_(b′)—

-   -   with b′/a′ in the range 0.1-40, preferably 0.5-20, a′ and b′        being integers such as to give the above molecular weight;        —(CF₂O)_(c′)(C₃F₆O)_(d)—    -   with c′/d′ in the range 0.01-5, c′ and d′ being integers such to        give the above molecular weight.

As oxidizing power it is meant the amount in grams of peroxidic oxygenper 100 grams of polymer.

As radical initiator, a peroxidic compound of formula (f) with the aboveoxidizing power is more preferably used. The preparation of theperoxidic initiators (f) can be carried out according to the processesdescribed in U.S. Pat. Nos. 3,847,978 and 5,488,181.

It has been found by the Applicant that both in the case of too lowoxidizing power and in the case of too high oxidizing power, thecrosslinking of the ionomeric chains is poor. The Applicant has foundthat using as initiator the compound of formula (f) having an oxidizingpower within the above values, a very good level of crosslinking isobtained.

When a radical initiatior of formula (f) is used for the crosslinking,it is fed in a concentration in the range 2%-10% by weight, preferably3-6% by weight with respect to the weight of the sulphonic ionomer.

As regards the crosslinking agent bis-olefin of formula (I), it is fedin a concentration in the range 3%-25% by weight, preferably 4-8% withrespect to the weight of the sulphonic ionomer.

According to a not binding theory, the Applicant keeps that in thecrosslinking initial phase, the radical initiator has the function toremove a proton from the bis-olefin of formula (I) present in the chainof the sulphonic ionomer. The so formed radical can attack the doublebond of the bis-olefin c) introduced as crosslinking agent and generatethe first part of the reticule. The repetition of this H* extractionevent on another ionomeric chain allows the reticule formation.

A further object of the invention are the supported or self-supportedmembranes, and their process of preparation by using the crosslinkedsulphonic fluorinated ionomers of the present invention.

For supported membranes, foamed PTFE, preferably bistretched, can beused as support, having a suitable dimension in connection with theelectrochemical cell, having a thickness comprised between 10 μm and 50μm. For the preparation of supported membranes, first a solution isprepared formed by:

-   -   A) a solution of the sulphonic ionomer in a fluorinated solvent        selected from: hexafluoroxylene, perfluorohexane,        perfluorooctane, perfluorobenzene, perfluoropolyether solvents,        fluoroether solvents.    -   B) a radical initiator of formula (d), (e) or (f);    -   C) the bis-olefin of formula (I) as crosslinking agent.

The porous support of foamed PTFE is dipped in the solution formed byA)+B)+C) for a time comprised between 10 seconds and 1 minute; theimpregnation is repeated more times until an impregnated membrane isobtained. Subsequently the ionomeric solution excess is removed from thesupport, for example by a roller system. The supported membrane obtainedby impregnation is then dried at 25° C. for about 1 hour, to remove thefluorinated solvent.

At this point, the membrane is crosslinked at the above mentionedtemperature, for a reaction time equal to 6 half-lives of the usedinitiator, i.e. until a decomposition of about 99% of the initiator.When a radical initiator of formula (f) is used, the crosslinkingaverage time generally ranges from about 10 seconds to 3 minutes.

Alternatively, the preparation of the supported membrane can be carriedout by casting. In the case of thick membrane (self-supported), themembrane can be obtained by casting or by press. In the latter case afilm of the solution A)+B)+C) is placed between two metal platescontaining a metal frame, which confers to the resulting membrane thedesired thickness. Then, the plate/frame/film system is put in a pressoven applying a weight of about 500-2,000 kg, preferably 750-1,500 Kg.

The supported or thick membranes, obtained from the sulphonic ionomerscrosslinked by the method of the present invention have a thicknessranging from 10 μm to 300 μm depending on the used preparation method.The membrane appears transparent, in some cases light brown-coloured.The morphological analysis at the microscope of the obtained membraneshows a substantially uniform, smooth and completely free from holessurface.

The obtained membrane, when it appears light brown-coloured, can bedecolorated. Decoloration can be carried out by dipping the membrane inthe —SO₂F form into an aqueous solution containing H₂O₂ at 9%(weight/volume) and HNO₃ at 10% (weight/volume) at 50° C. for about 4-6hours. A colourless and transparent membrane is obtained with a maximumweight loss lower than 1% by weight (colour loss higher than 95% by UVanalysis). The decoloration treatment can be carried out both before andafter the membrane activation treatment described hereunder.

The membrane is then subjected to the activation treatment fortransforming the sulphonyl groups —SO₂F into sulphonic groups —SO₃H. Theactivation implies 2 steps:

-   -   salification for transforming the —SO₂F form into the —SO₃ ⁻        form;    -   acidification for transforming the —SO₃ ⁻ form into the —SO₃H        form.

Salification is carried out by dipping the membrane obtained after thecrosslinking reaction in a basic aqueous solution of KOH or of NaOH at atemperature such as to have an almost total conversion into the —SO₃ ⁻groups.

It has been found by the Applicant that, for equivalent weights lowerthan 800, a temperature in the range 5° C.-40° C., for a time comprisedbetween 4 and 40 hours, can be used. The 97% of the conversion takesplace in the first 20-30 minutes of reaction; further 5 hours arenecessary for obtaining the required conversion of 99.9% (upper limit ofinstrumental detection). The weight loss due to the dissolving of thesalified membrane in water is lower than 20%. Temperatures higher than40° C., shorten the conversion times, but they remarkably increase theweight loss of the membrane (for example operating at 70° C., there is aweight loss equal to 90-95%). Therefore, the crosslinked membranesobtained from the sulphonic fluorinated ionomers having a low equivalentweight of the invention show the additional advantage to be salified atroom temperature.

At the end of the salification, the membrane is dipped into a distilledwater bath at 25° C. for washing the residual base.

The acidification is carried out by dipping the salified membrane in anaqueous solution containing the 20% by weight of HCl at 25° C. for 5hours. The conversion is equal to 99.9% 20 with a weight loss of themembrane in the —SO₃H form lower than 1%. The resulting crosslinked,optionally decolorated, membrane in the —SO₃H form, is suitable to beused in applications of electrochemical type, for example of fuel celltype or in catalysis applications as ionic exchange resin.

The crosslinked membranes of the invention after activation, i.e. in the—SO₃H form, show the following propeties:

-   -   amount of gels higher than 85%, generally higher than 90%;    -   after drying and hydration in water at 100° C. for 30 minutes,        the membrane substantially remains integer without dissolving in        water.

As “gel” is meant the insoluble part of the polymer and the % by weightof gels is correlated with the crosslinking degree of the ionomeritself. Indicatively, the higher the % by weight of gels, the lower theamount of uncrosslinked ionomer.

Tests carried out by the Applicant (see the Examples) show that theobtained membranes, notwithstanding the high hydration percentage, havea good physical integrity and they are substantially insoluble in waterboth at low and at high temperature (up to about 180° C.). Inparticular, the T_(r) values (interstitial water release temperature)obtained by thermogravimetric analysis TGA, result surprisingly high.

The interstitial water is the carrier of the H⁺ protons in fuel cells,wherefore the invention membranes, having a T_(r) in the range 160°C.-180° C., are capable to mantain a good conductivity also at very hightemperatures, for example of the order of about 160° C. This issurprising and unexpected since the membranes of sulphonic ionomersknown in the prior art are not usable in an efficient way attemperatures higher than 100° C.

Besides in the preparation of membranes for fuel cells, the sulphonicionomers of the present invention can successfully be used in thepreparation of ionic exchange resins for carrying out chemico-physicalseparations and as acid catalyst for chemical reactions. The crosslinkedionomer of the invention, when used as an acid catalyst, appearsextremely effective since it allows to obtain high yields in shorttimes.

As regards the preparation of crosslinkable sulphonic fluorinatedionomers, it can be carried out by polymerization in aqueous emulsionaccording to well known methods of the prior art, in the presence ofradical initiators (for example, alkaline or ammonium persulphates,perphosphates, perborates or percarbonates), optionally in combinationwith ferrous, cupric or silver salts, or other easily oxidizable metals.In the reaction medium also surfactants of various type are usuallypresent, among which the fluorinated surfactants of formula:R_(f)—X⁻ M⁺are particularly preferred, wherein R_(f) is a C₅-C₁₆ (per)fluoroalkylchain or a (per)fluoropolyoxyalkylene chain, X⁻ is —COO⁻ or —SO₃ ⁻, M⁺is selected from: H⁺, NH₄ ⁺, alkaline metal ion. Among the most commonlyused we remember: ammonium perfluoro-octanoate, (per)fluoropolyoxyalkylenes ended with one or more carboxylic groups, etc.

When the polymerization is over, the ionomer is isolated by conventionalmethods, such as coagulation by addition of electrolytes or by cooling.

Alternatively, the polymerization reaction can be carried out in bulk orin suspension, in an organic liquid wherein a suitable radical initiatoris present, according to well known techniques.

The polymerization reaction is generally carried out at temperatures inthe range 25°-150° C., under pressure up to 10 MPa.

The preparation of the sulphonic fluorinated ionomers of the inventionis preferably carried out in aqueous emulsion in the presence of anemulsion, dispersion or microemulsion of perfluoropolyoxyalkylenes,according to U.S. Pat. No. 4,789,717 and U.S. Pat. No. 4,864,006.

The present invention will be now better illustrated by the followingembodiment Examples, which have a merely indicative purpose but notlimitative of the scope of the invention itself.

EXAMPLES

Characterization

Percentage of Gels

The “gel” term is defined as the insoluble part of a polymer in the samesolvent wherein it was soluble before the crosslinking. The % of gels istherefore correlated with the crosslinking degree of the polymer itself.

The gel % is evaluated before and after the crosslinking by thefollowing procedure:

-   -   the ionomer is solubilized in perfluorohexane with an        ionomer/solvent concentration of about 25% (weight/volume);    -   the solution is left under stirring at 40° C. for 24 hours; at        the end the % by weight of ionomer which is not dissolved is        evaluated.        Percentage of Hydration

After drying (1 hour at 50 ° C.), the membrane is weighed andsubsequently hydrated in distilled water at 100° C. for 30 minutes; thenit is extracted from water, dried on the surface and weighed again.

The hydration percentage H % of the membrane is evaluated according tothe following formula:H %=100×(hydrated membrane weight-dried membrane weight-/dried membraneweight)Percentage of Extractable Substances

The dried membrane is first weighed and then put in a solution ofethanol/water 40/60 by weight at 50° C. for 22 hours. Subsequently thesolution is filtered on filter Whatman 541. The filtered product isdried at 80° C. and the dry residue is weighed.

The percentage of extractable substances E % is calculated according tothe following formula:E %=100×(dry residue weight/membrane initial weight)

The lower the percentage of extractable E %, the higher thechemico-physical resistance of the obtained membrane.

Release Temperature of the Interstitial Water

The release temperature T_(r) of the interstitial water is evaluated bythermogravimetric analysis (TGA).

An amount of about 10 mg of the membrane, hydrated in distilled water at100° C. for 30 minutes, is analyzed in a thermogravimetric analyserPerkin Elmer model TGA7. After having been maintained in N₂ flow at roomtemperature, the sample is heated with a temperature gradient of 10°C./min up to 80° C., temperature at which it is maintained for 5minutes. Subsequently the sample is cooled at the same rate until roomtemperature, at which it is maintained for 10 minutes. Then thethermogravimetric analysis starts by heating the sample at a rate of 10°C./min.

The TGA curve derivative allows to go back to the value of the releasetemperature T_(r) in correspondence of the second minimum of thederivative of the TGA curve. The higher the temperature T_(r), thegreater the capability of the membrane to retain water and consequentlyto lead also to high temperatures.

Example 1

In a 2 litre autoclave, the following reactants are introduced:

-   -   700 ml of demineralized water;    -   45 ml of the monomer of formula CF₂═CF—O—CF₂CF₂—SO₂F;    -   29 g of a microemulsion of perfluoropolyoxyalkylenes previously        obtained by mixing:        -   11.6 g of a perfluoropolyoxyalkylene having a potassium            salified acid end group of formula:            ClCF₂O(CF₂—CF(CF₃)O)_(n)(CF₂O)_(m)CF₂COOK        -   wherein n/m=10 having number average molecular weight 527;        -   5.8 g of a perfluoropolyether oil Galden® D02 of formula            CF₃O(CF₂—CF(CF₃)O)_(n)(CF₂O)_(m)CF₃        -   wherein n/m=20 having average molecular weight 450        -   11.6 g of water;    -   240 g of a bis-olefin of formula CH₂═CH—(CF₂)₆—CH═CH₂ fed in a        Galden® D02 solvent obtaining a solution at 30% by volume of        bis-olefin.

The autoclave, kept under stirring at 700 rpm, has been heated up to 50°C. 400 ml of an aqueous solution having a concentration of 20 g/l ofpotassium persulphate (KPS) are then fed into the autoclave. Thepressure is brought to 3 absolute bar introducing TFE. The reactionstarts after 3 min. The pressure is maintained at 3 absolute bar byfeeding TFE.

The total mass of TFE fed to the reactor is equal to 760 g. The totalmass of sulphonyl monomer CF₂═CF—O—CF₂CF₂—SO₂F fed to the reactor isequal to 2,126 g.

The reaction is stopped after 280 min from the starting, lessening thestirring, cooling the reactor and venting the TFE. The produced latexhas a solid content of 25% by weight. The latex is coagulated byfreezing, the polymer is separated from the mother liquors and dried at100° C. for 8 h at room pressure. The composition by moles of thecopolymer determined by NMR results to be:

-   -   62.9% TFE, 35.1% sulphonic monomer, 2% bis-olefin. The        equivalent weight results of 480 g/eq.

On a sample of sulphonic ionomer obtained from the polymerization, thegel % is evaluated according to the above procedure. A value of gelsequal to 10% is obtained and the ionomer almost completely dissolves insolution.

Crosslinking

17.64 g of ionomer obtained by the above described polymerization aredissolved in 82.36 g of a perfluoropolyether solvent of formulaCF₃CF₂O—(C₃F₆O)₃—CF₂H obtaining an ionomeric solution having an ionomerconcentration equal to 17.64% by weight.

To 100 g of the ionomeric solution:

-   -   5% by weight referred to the ionomer of the bis-olefin of        formula CH₂═CH—(CF₂)₆—CH═CH₂;    -   4% by weight of an initiator having the formula:        T-O-—(R_(f))—(O)_(c)-T′        are added        wherein: T,T′=-CF₃    -   c is an integer such to give an oxidizing power equal to 2;    -   R_(f)=perfluoropolyether chain having a number average molecular        weight in the range 6,500-7,500, and structure:        —(CF₂O)_(a′)(CF₂CF₂O)_(b′)—        with b′/a′ in the range 0.5 and 20, a′ and b′ being integers        such as to give the above indicated molecular weight.

A mother solution containing sulphonic ionomer, bis-olefin and radicalinitiator is obtained.

85% of the solvent is evaporated under light nitrogen flow. Theremaining 15% of the solvent is evaporated, leaving the mother solutionunder ventilation of an inert gas. An homogeneous film formed byionomer, bis-olefin and radical initiator is thus obtained.

Crosslinking of the ionomeric film is carried out a temperature of 270°C. The reaction time is equal to 35 seconds. After crosslinking thepercentage by weight of gels on the basis of the above procedure isevaluated. A gel value equal to 98% is obtained. The ionomer can thenface the subsequent activation reaction for transforming the sulphonylgroups —SO₂F into sulphonic groups —SO₃H without substantial materialloss.

Example 2

Example 1 is repeated with the difference that the used amount ofbis-olefin of formula CH₂═CH—(CF₂)₆—CH═CH₂ is equal to 15% by weightwith respect to the sulphonic ionomer. A mother solution containingsulphonic ionomer, bis-olefin and radical initiator is obtained.

85% of the solvent is evaporated under light nitrogen flow. Theremaining 15% of the solvent is evaporated by leaving the mothersolution under ventilation of an inert gas. An homogeneous film formedby ionomer, bis-olefin and radical initiator is thus obtained.

Crosslinking of the ionomeric film is carried out a temperature of270C.°. The reaction time is equal to 35 seconds. After crosslinking agel value equal to 96% is obtained.

Example 3

Example 1 is repeated but using the initiator of Example 1 in an amountequal to 3% by weight with respect to the ionomer. The crosslinkingprocedure of Example 1 is repeated. After crosslinking according to themethod of the present invention, a gel value equal to 90% is obtained.

Example 4

Example 1 is repeated, using an initiator having the same structure ofthat of Example 1, but with an oxidizing power equal to 3.5

The crosslinking temperature is equal to 290° C.

After crosslinking according to the method of the present invention, agel value equal to 86% is obtained.

Example 5 (Comparative)

Example 1 is repeated, but carrying out the crosslinking at thetemperature of 210° C.

After a reaction time equal to 30 minutes the percentage of gels isevaluated. A gel value equal to 10% is obtained. This Example shows thatat the temperature of 210° C. the crosslinking reaction of the inventiondoes not take place.

Example 6 (Comparative)

Examaple 1 is repeated with the difference that the used amount ofbis-olefin of formula CH₂═CH—(CF₂)₆—CH=CH₂ is equal to 2% by weight withrespect to the sulphonic ionomer. The crosslinking procedure of Example1 is repeated. At the end of the crosslinking the gel % results equal to60%. This Example shows that when the % by weight of crosslnking agent(bis-olefin) is lower than the defined range, the crosslinking reactionof the invention only partially takes place.

Example 7 (Comparative)

Example 1 is repeated, using an initiator having the same structure ofthat of Example 1, but with an oxidizing power equal to 0.4, in anamount of 10% by weight with respect to the ionomer.

The crosslinking procedure of Example 1 is repeated.

The gel % results equal to 10%. This Example shows that when theoxidizing power of the initiator is lower than 0.8, the crosslinkingreaction of the invention does not take place.

Example 8

Activation

The film obtained by the crosslinking carried out in Example 1 issubjected to a salification treatment for 6 hours at 25° C. in a KOHaqueous solution at 10% by weight. The 97% of the conversion takes placein the first 20-30 minutes of reaction; further 5 hours are insteadnecessary for obtaining the required conversion of 99.9% (upper limit ofinstrumental detection). The weight loss due to the dissolving of thesalified membrane in water is equal to 14%.

At the end of the salification, the membrane is dipped into a distilledwater bath at 25° C. for washing the residual KOH.

The acidification is carried out by placing the salified membrane in anaqueous solution containing the 20% by weight of HCl at 25° C. for 5hours. The conversion is equal to 99.9% with a weight loss of themembrane in the —SO₃H form lower than 1%.

The previous treatment of salification and acidification involves thecomplete transformation, in the limits detectable at the FTIR analysis,of the —SO₂F groups into sulphonic groups —SO₃H.

This Example shows that the crosslinked film according to the method ofthe present invention can advantageously be salified at roomtemperature.

Example 9

A membrane is prepared by using as a support, bistretched foamed PTFEhaving thickness of 35 μm and porosity equal to 0.2 μm.

17.64 g of ionomer obtained by the polymerization of Example 1 aredissolved in 82.36 g of a perfluoropolyether solvent of formulaCF₃CF₂O—(C₃F₆O)₃—CF₂H obtaining an ionomeric solution having an ionomerconcentration equal to 17.64% by weight.

To 100 g of the ionomeric solution:

-   -   5% by weight referred to the ionomer of the bis-olefin of        Example 1;    -   4% by weight of the radical initiator of Example 1; are added.

A mother solution containing sulphonic ionomer, bis-olefin and radicalinitiator is obtained. The porous support of foamed PTFE is dipped intothis solution for 10 seconds; the impregnation is repeated 3 times,until a 100% impregnated membrane, completely transparent, havingthickness of 110 μm, is obtained. Subsequently the ionomeric solution inexcess is removed from the support by a roller system. The supportedmembrane obtained from the impregnation is then dried at 50° C. forabout 1 hour, to eliminate the fluorinated solvent.

At this point, the membrane is crosslinked at the temperatue of 270° C.for a reaction time equal to 35 seconds. Then the membrane is activatedin the —SO₃H form following the procedure described in Example 8. Theweight loss % during the activation is equal to 12.6%.

A percentage of extractable substances E % equal to 2% is determined.

The percentage of hydration H % is equal to 40%.

The release temperature T_(r) of the interstitial water is 175° C.

Example 10

A membrane is prepared following the same impregnation procedure ofExample 9, but using as a support monostretched foamed PTFE havingthickness of 60 μm and porosity equal to 0.45 μm.

As solvent for the ionomeric solution CH₃OC₄F₉ (HFE 7100) is used.Crosslinking takes place at the temperature of 270° C. for a reactiontime equal to 35 seconds. After crosslinking, the membrane is activatedin the —SO₃H form following the procedure of Example 8.

The so obtained ionomeric membrane, having thickness of 65 μm, is driedfor 1 h at 50° C.

The percentage of extractable subsatances E % is 20%.

The percentage of hydration H % is equal to 40%.

The release temperature T_(r) of the interstitial water is 168° C.

Example 11

A non supported thick membrane is prepared by casting from solvent,using the same perfluoropolyether solvent of Example 9. Crosslinkingtakes place at the temperature of 270° C. for a reaction time equal to35 seconds. After crosslinking, the membrane is activated in the —SO₃Hform following the procedure of Example 8. The weight loss % during theactivation is equal to 12.6%.

The so obtained ionomeric membrane, having thickness of 250 μm, is driedfor 1 h at 50° C.

The percentage of extractable substances E % is 21%.

The percentage of hydration H % is equal to 55%.

The release temperature T_(r) of the interstitial water is 161° C.

Example 12

A supported membrane is prepared using as a support bi-stretched foamedPTFE having thickness of 35 μm and porosity equal to 0.2 μm. Theprocedure by casting is used, using the same perfluoropolyether solventof Example 9.

Crosslinking takes place at the temperature of 270° C. for a reactiontime equal to 35 seconds. After crosslinking, the membrane is activatedin the —SO₃H form following the procedure of Example 8.

The so obtained ionomeric membrane, having thickness of 65 μm, is driedfor 1 h at 50° C.

The percentage of extractable substances E % is 11%.

The percentage of hydration H % is equal to 92%.

The release temperature T_(r) of the interstitial water is 164° C.

Example 13

In this Example the membrane obtained in Example 9 is used as acidcatalyst in the Friedel-Craft acylation reaction. The Friedel-Craftreaction has been used as a test for evaluating the catalysis efficiencyof the sulphonic groups present in the membrane.

The 3% by weight of the membrane in the —SO₃H form of Example 9 is addedto an equimolar mixture of the reactants anisole and acetic anhydridewith respect to the anisole.

The reaction formation of the reaction product 4-inethoxyacetophenone,carried out at a temperature of 22° C., reaches a conversion of 95% in25 minutes and it is completed with a conversion of 100% in 37 minutes.

This Example shows that the crosslinked sulphonic fluorinated ionomersby radical route of the invention can advantageously be used as acidcatalyst for chemical reactions. TABLE 1 Crosslinking InitiatorInitiator Temperature Reaction Gels agent (%) (%) PO (° C.) time (%)Example 1 5 4 2.0 270 35 seconds 98 Example 2 15 4 2.0 270 35 seconds 96Example 3 5 3 2.0 270 35 seconds 90 Example 4 5 4 3.5 290 35 seconds 86Example 5 Comp. 5 4 2.0 210 30 minutes 10 Example 6 Comp. 2 4 2.0 270 35seconds 60 Example 7 Comp. 5 15 0.4 270 35 seconds 10

TABLE 2 Membrane Extractable thickness substances Hydration Tr SupportMethodology (μm) (%) (%) (° C.) Example 9 bistretched impregnation 110 240 175 foamed PTFE Example 10 monostretched impregnation 65 20 40 168foamed PTFE Example 11 — casting 250 21 55 161 Example 12 bistretchedcasting 65 11 92 164 foamed PTFE

1. A process for preparing sulphonic fluorinated ionomers comprisingradical crosslinking of: A) crosslinkable sulphonic fluorinatedionomers, having equivalent weight 380-1300 g/eq and comprising: from48% to 85% by moles of monomeric units deriving from tetrafluoroethylene(TFE); from 15% to 47% by moles of fluorinated monomeric unitscontaining sulphonyl groups —SO₂F; from 0.01% to 5% by moles ofmonomeric units deriving from a bis-olefin of formula:

wherein: m=2-10; R₁, R₂, R₅, R₆, equal to or different from each other,are H or C₁-C₅ alkyl groups; B) a fluorinated compound as crosslinkingradical initiator; C) a fluorinated bis-olefin of structure (I) ascross-linking agent; wherein the radical crosslinking is carried out ata temperature in the range of 250° C.-310° C.
 2. The process accordingto claim 1, wherein the fluorinated monomers containing sulphonyl groups—SO₂F are selected from the group consisting of:F₂C═CF—O—CF₂—CF₂—SO₂F;F₂C═CF—O—(CF₂—CXF—O)_(n,)—CF₂—CF₂—SO₂F wherein X═Cl, F or CF₃; n′=1-10;F₂C═CF—O—CF₂—CF₂—CF₂—SO₂F (vinylsulphonylfluoride); F₂C═CF—Ar—SO₂Fwherein Ar is an aryl ring.
 3. The process according to claim 1, whereinthe crosslinkable sulphonic fluorinated ionomers A) comprise: from 54%to 71% by moles of monomeric units deriving from TFE; from 45% to 28% bymoles of monomeric units deriving from vinylsulphonylfluorideF₂C═CF—O—CF₂—CF₂═SO₂F; an amount higher than 0.4% by moles up to 3% bymoles of monomeric units deriving from the bis-olefin of formula (I). 4.The process according to claim 1, wherein the radical initiators B) areselected from the group consisting of: (d) branched perfluoralkanes offormula:C_(a)F_(2a+2) wherein a=5-15; (e) halogenated compounds of formula:ClO₂S(CF₂)_(n)SO₂Cl wherein n=4-10; (f) peroxidic perfluoropolyethercompounds having oxidizing power in the range 0.8-6 of structureT-O—(R_(f))—(O)_(c)-T′ wherein:T,T′=-CF₃,—CF₂CF₃, —CF₂CF₂CF₃ c is an integer such as to give the aboveoxidizing power; R_(f) is a perfluoropolyether chain having a numberaverage molecular weight in the range 1,000 and 30,000, comprising oneor more of the following units:—(CF₂O)—, —(CF₂CF₂O)—, —(CF₂CF₂CF₂O)—, —(C₃F₆O)—.
 5. The processaccording to claim 4, wherein the perfluoropolyether chain R_(f) isselected from the following structures:—(CF₂O)_(a), (CF₂CF₂O)_(b′)— with b′/a′ in the range 0.1-40, a′ and b′being integers such as to give the above molecular weight;—(CF₂O)_(c), (C₃F₆O)_(d′)— with c′/d′ in the range 0.01-5, c′ and d′being integers such to give the above molecular weight.
 6. The processaccording to claim 4, wherein a radical initiator of formula (f) is usedin a concentration in the range 2%-10% by weight, with respect to theweight of the sulphonic ionomer.
 7. The process according to claim 1,wherein as crosslinking agent the bis-olefin of formula (I) is used in aconcentration in the range of 3%-25% by weight with respect to theweight of the sulphonic ionomer.
 8. A process for preparing supported orself-supported membranes comprising utilizing the sulphonic fluorinatedionomers crosslinked according to the process of claim 1 to prepare thesupported or self-supported membranes.
 9. The process according to claim8, wherein the support is foamed PTFE.
 10. The process according toclaim 9, comprising preparing a solution formed by: A) a solution of thesulphonic ionomer in a fluorinated solvent selected from the groupconsisting of: hexafluoroxylene, perfluorohexane, perfluorooctane,perfluorobenzene, perfluoropolyether solvents, fluoroether solvents: B)a radical initiator of formula (d), (e) or (f); (d) branchedperfluoralkanes of formula:C_(a)F_(2a+2) wherein a=5-15; (e) halogenated compounds of formula:ClO₂S(CF₂)_(n)SO₂Cl wherein n=4-10; (f) peroxidic perfluoropolyethercompounds having oxidizing power in the range 0.8-6 of structureT-O—(R_(f))—(O)_(c)-T′ wherein:T,T′=-CF₃,—CF₂CF₃, —CF₂CF₂CF₃ c is an integer such as to give the aboveoxidizing power; R_(f) is a perfluoropolyether chain having a numberaverage molecular weight in the range 1,000 and 30,000, comprising oneor more of the following units:—(CF₂O)—, —(CF₂CF₂O)—, —(CF₂CF₂CF₂O)—, —(C₃F₆O)—; C) the bis-olefin offormula (I) as crosslinking agent; and dipping a porous support offoamed PTFE in the solution formed by A)+B)+C) for a time comprisesbetween 10 seconds and 1 minute and repeating the dipping more times;the impregnated membrane being crosslinked at the crosslinkingtemperature for a reaction time equal to 6 half-lives of the usedinitiator.
 11. The process according to claim 8, comprising casting themembranes from solvent.
 12. The process according to claim 8, comprisingcasting or pressing the membranes.
 13. The process according to claim12, comprising forming a film of the solution: A) a solution of thesulphonic ionomer in a fluorinated solvent selected from the groupconsisting of: hexafluoroxylene, perfluorohexane, perfluorooctane,perfluorobenzene, perfluoropolyether solvents, fluoroether solvents; B)a radical initiator of formula (d), (e) or (f): (d) branchedperfluoralkanes of formula:C_(a)F_(2a+2) wherein a=5-15; (e) halogenated compounds of formula:ClO₂S(CF₂)_(n)SO₂Cl wherein n=4-10; (f) -peroxidic perfluoropolyethercompounds having oxidizing power in the range 0.8-6 of structureT-O—(R_(f))—(O)_(c)-T′ wherein:T,T′=-CF₃,—CF₂CF₃, —CF₂CF₂CF₃ c is an integer such as to give the aboveoxidizing power; R_(f) is a perfluoropolyether chain having a numberaverage molecular weight in the range 1,000 and 30,000, comprising oneor more of the following units:—(CF₂O)—, —(CF₂CF₂O)—, —(CF₂CF₂CF₂O)—, —(C₃F₆O)—; C) the bis-olefin offormula (I) as crosslinking agent; and placing the film between twometal plates containing a metal frame; and thereafter putting theplate/frame/film system in a press oven and applying a weight of about500-2,000 kg.
 14. The process according to claim 8, wherein thesulphonyl groups —SO₂F are transformed into sulphonic groups —SO₃H by 2steps: salification for transforming the —SO₂F form into the —SO₃K form;acidification for transforming the —SO₃K form into the —SO₃H form. 15.The process according to claim 14, wherein the salification is carriedout in a basic aqueous solution of KOH or of NaOH at a temperature inthe range 5° C.-40° C. for a time comprises between 4 and 40 hours. 16.The process according to claim 14, wherein the acidification is carriedout in an aqueous solution containing the 20% by weight of HCl at 25° C.for 5 hours.